The present invention relates to a method for producing an oxygen detection element, and, more particularly, to a method for producing an oxygen detection element having an excellent durability against toxic matter in the measurement gas brought about by providing the detection element with electrodes having uniform pores. The present invention is applied to oxygen detection elements for measuring oxygen concentration in exhaust gases from internal engines, various kinds of combustors, and the like. More particularly, the present invention is applicable to zirconia lambda sensors, zirconia air-fuel ratio sensors, and the like.
A conventional method for producing an oxygen detection element includes steps of: preparing an unsintered oxygen-ion conductive solid-state electrolytic body; applying a paste containing a fine powder-like ceramic material, a fine powder-like catalytic material for effecting gas equilibrium, and diluent oil onto at least one portion of the outer surface of the solid-state electrolytic body; sintering the body having the paste thereon to produce an electrode on the solid-state electrolytic body in the form of a catalytic layer having pores; and forming an outer protective layer covering the electrode. (See, for example, Japanese Patent Publication No. 59-24382.) The element produced by this method exhibits an abrupt change in output voltage as the stoichiometric ratio of the measurement gas is crossed.
However, the main components for forming the electrode by the conventional method include noble metal fine particles and ceramic fine particles. Accordingly, a large amount of ceramic material (for example, five times or more than the amount of the catalytic material) must be added to the catalytic material to bring the particles sufficiently into contact with each other to thereby prevent self sintering of the catalytic material and to form electrodes having pores. In this case, the electrical conductivity of the electrodes is apt to be reduced. Further, the contact area between the solid-state electrolyte and the catalytic material forming the electrode on the surface of the solid-state electrolyte is apt to be reduced. Thus, there arises a problem in that the .intern 1 resistance of the electrode increases, making it necessary to increase the thickness of the coating of catalytic material.
Furthermore, since the aforementioned two kinds of powder are made of different materials, for example, noble metal and ceramics, it is difficult to mix them uniformly. Even if they are mixed sufficiently, a portion which has an insufficient dispersion of ceramic powder may remain because of the acidity or alkalinity of the ceramic powder (depending on the kind of solvent used), as a result of which a nonuniform microscopic structure is apt to be formed. Accordingly, pores can be formed which are so large that gas directly reaches the three-phase boundary between the electrode and the solid-state electrolyte.
As described above, reducing the contact area brings about a reduction of the three-phase boundary, and, at the same time, the large pores result in direct contact by the gas. Accordingly, when the oxygen detection element is used in a gas containing toxic matter such as silicon, sulfur, lead or the like, the toxic matter may bring cause an abnormality in the characteristics of the sensor in a short time.
Further, there is a difference in melting point among the materials of which the paste is formed. Zirconium oxide (generally used in the solid-state electrolyte) or aluminum oxide (present as an impurity) has a higher melting point than that of platinum, rhodium, palladium or the like. Thus, self-sintering of the noble metal powder occurs if a sufficient quantity of noble metal powder is not added.
Therefore, an electrode formed by applying and sintering a paste prepared as a mixture of powdered materials is not satisfactory in practical use.